In recent years, great attention has been drawn to a fiber laser system which is configured to achieve an output power of several kW (kilowatt) by combining laser beams, which are emitted from a respective plurality of fiber lasers and each of which is substantially a single mode, so as to obtain and output a multi-mode light beam. A fiber laser is a laser device which includes, as a laser medium, an optical fiber having a core to which a rare earth element is added. Such a high-power fiber laser system is mainly used in the field of material processing.
FIG. 11 is a view illustrating a specific configuration of the fiber laser system. As illustrated in FIG. 11, a fiber laser system 10 includes (i) a plurality of fiber laser units (FLU) 20, (ii) an output combiner 30, (iii) an output section 40, (iv) optical fibers f1 which connect the FLUs 20 to the output combiner 30, and (v) an optical fiber f2 which connects the output combiner 30 to the output section 40.
Although not illustrated in FIG. 11, each of the FLUs 20 includes (i) an excitation light source which outputs multi-mode excitation light and (ii) an amplifying optical fiber which amplifies the excitation light so as to obtain and output a laser beam which is substantially a single mode.
The output combiner 30 generates a multi-mode laser beam by combining the laser beams which are outputted from the respective FLUs 20 and each of which is substantially a single mode. The multi-mode laser beam outputted from the output combiner 30 propagates through the optical fiber f2, is converted by the output section 40 into a laser beam L10 (hereinafter referred to as “beam L10”) for processing, and in a case where a processing object 50 (i.e. an object to be processed) is provided, is focused on the processing object 50.
Optical fibers (amplifying optical fibers of the FLUs 20, the optical fibers f1, and the optical fiber f2) included in the fiber laser system 10 guide reflected light which results from Rayleigh scattering caused inside the optical fibers, as well as the laser beams (laser beam ultimately outputted as the beam L10) amplified by the amplifying optical fibers. In a case where the fiber laser system 10 is used for material processing, that is, in a case where the beam L10 is emitted toward the processing object 50, part of reflected light L11, which is light obtained when the beam L10 is reflected from the processing object 50, may be returned into the fiber laser system 10 through an output end of the output section 40. In this case, the optical fibers guides also the part of the reflected light L11 which part has been returned to the fiber laser system 10.
It is known that in a case where an output of the beam L10 is increased in the fiber laser system 10 thus configured, a power of light guided through each of the optical fibers is increased so as to cause stimulated Raman scattering (SRS). SRS is a kind of nonlinear optical effect which occurs in a core of each of the optical fibers, wherein the core serves as a medium of SRS. SRS can be regarded as a process of power conversion from a laser beam to a Stokes beam. Particularly in the optical fiber f2 provided so as to follow the output combiner 30, SRS occurs with a high Raman gain (conversion efficiency in the process of power conversion), since the optical fiber f2 guides a laser beam which is obtained by combining the laser beams outputted from the three FLUs 20 and which accordingly has a significantly high power.
As a Raman gain increases, a power of a Stokes beam caused by SRS increases. It is known that the FLUs 20 accordingly have a less stable oscillation state, and consequently invite a failure of the fiber laser system 10 (see Patent Literature 1). In view of this, in the fiber laser system 10, destabilization of oscillation states of the FLUs 20 is prevented by setting a length and core diameter of each of the optical fibers and a power of each of the FLUs so that a power of a Stokes beam relative to a power of the beam L10 is below a certain reference level during steady operation. The reference level differs depending on a reflection resistance which the fiber laser system 10 is expected to have, but the reference level is set to be, for example, −50 dB when processing is carried out.